Receivers for solar concentrators currently have a metallic tube into which the heat transfer fluid flows; the concentrator mirror reflects solar radiation, concentrating it on the tube, in which the fluid is heated in order to be used as a heat source for industrial processes or for the generation of electric power in a thermodynamic-cycle plant, optionally of the combined-cycle type for the combined generation of electric power and thermal power.
One strongly felt need in the field of solar concentration plants is to optimize the efficiency of the conversion of the absorbed solar energy into heat of the heat transfer fluid, in particular by minimizing heat losses.
Such losses have a radiative component, a conductive component and a convective component; the latter two are influenced by the characteristics of the environment surrounding the tube.
In order to contrast radiative dissipations, the tube is covered with a selective covering that is capable of allowing efficient absorption by the tube of radiation with a wavelength substantially comprised between 320 nm and 2000 nm while having a low emissivity of radiation with a wavelength of more than 2000 nm, which correspond to infrared radiation.
Furthermore, in order to contrast heat dissipation from the tube by conduction and convection, the tube is enclosed in a glass shell that defines, between itself and the tube, a gap in which vacuum is produced, i.e., in which there is air at a pressure substantially equal to 10−4 mbar.
Gas absorbers, also known as getters, adapted to eliminate the gases that have penetrated into the gap through the walls, are generally arranged in the gap.
Currently known receivers are composed of modules that comprise a portion of tube that is accommodated in a corresponding portion of glass shell, to which the tube is sealed at the ends by means of hermetic connectors which are provided with a deformable accordion-like portion for hermetic connection of the tube to the shell so as to not hinder their differential expansion, particularly in a longitudinal direction.
The modules are joined end to end at the connectors by welding, so that the portions of tube define a continuous duct for the heat transfer fluid.
The extension of the connectors is considerable with respect to the length of the module in order to allow adaptation to the considerable longitudinal differential expansion that the tube and the glass shell undergo during the operation of the concentrator.
Nowadays synthetic oil is generally used as a heat transfer fluid and has operating temperatures that are generally lower than, or equal to, 400° C.
In some recently developed plants, mixtures of molten nitrate salts are used instead at operating temperatures up to approximately 550° C.
As is known, the heat loss that occurs at the tube during the operation of the concentrator increases as the temperature of the tube rises.
In plants that comprise currently known receivers and use synthetic oil as a heat transfer fluid, the concentrator heats the oil to a temperature that is lower than approximately 400° C., achieving an average receiver efficiency of 79%.
In other plants currently being developed, which use molten salts as a heat transfer fluid, the fluid is brought, in currently known receivers, to an operating temperature of approximately 550° C., fully to the advantage of the efficiency of the user thermodynamic-cycle plant, yet making it possible to achieve a receiver efficiency of approximately 70%.
The efficiency losses that occur when using this second solution with respect to the first one are mainly due to the heat dissipations located at the receiver.
In particular, among the heat dissipation modes that underlie the efficiency reduction linked to the increase in the operating temperature of the heat transfer fluid, the radiation mode is predominant and is known to increase with the fourth power of the temperature of the heat transfer fluid.